A method is known in which when applying to a plurality of LED blocks a rectified voltage that a diode bridge outputs by full-wave rectifying the AC power supplied from a commercial power supply, the connection mode of the plurality of LED blocks is switched between a parallel connection and a series connection in accordance with the supply voltage (refer, for example, to patent document 1).
LEDs have nonlinear characteristics such that, when the voltage being applied across the LED reaches or exceeds its forward voltage drop, a current suddenly begins to flow. Light with a desired luminous intensity is produced by flowing a prescribed forward current (If) using a method that inserts a current limiting resistor or that forms a constant current circuit using some other kind of active device. The forward voltage drop that occurs is the forward voltage (Vf). Accordingly, in the case of a plurality, n, of LEDs connected in series, the plurality of LEDs emit light when a voltage equal to or greater than n×Vf is applied across the plurality of LEDs. On the other hand, the rectified voltage that the diode bridge outputs by full-wave rectifying the AC power supplied from the commercial power supply varies between 0 (v) and the maximum output voltage periodically at a frequency twice the frequency of the commercial power supply. This means that the plurality of LEDs emit light only when the rectified voltage is equal to or greater than n×Vf (v), but do not emit light when the voltage is less than n×Vf (v).
To address this deficiency, two LED blocks, each containing n LEDs, for example, are provided and, when the supply voltage reaches or exceeds 2×n×Vf (v), the two LED blocks are connected in series, causing the LEDs in both blocks to emit light; on the other hand, when the supply voltage is less than 2×n×Vf (v), the two LED blocks are connected in parallel so as to cause the LEDs in both blocks to emit light. By thus switching the connection of the plurality of LED blocks between the series connection and the parallel connection in accordance with the supplied voltage, the light-emission period of the LEDs can be lengthened despite the variation of the commercial power supply voltage.
However, since this method requires the provision of a switch circuit for switching the connection mode of the plurality of LED blocks, there has been the problem that not only does the overall size and cost of the LED driving circuit increase, but the power consumption also increases because of the power required to drive the switch circuit. In particular, if the light-emission period of the LEDs is to be further lengthened, the number of LED blocks has to be increased, but if the number of LED blocks is increased, the number of switch circuits required correspondingly increases.
Further, the switching timing of the switch circuit is set based on the predicted value of n×Vf (v), but since Vf somewhat varies from LED to LED, the actual value of n×Vf (v) of each LED block differs from the preset value of n×Vf (v). This has led to the problem that even if the switch circuit is set to operate in accordance with the supply voltage, the LEDs in both blocks may not emit light as expected, or conversely, even if the switching is made earlier than the preset timing, the LEDs may emit light; hence, the difficulty in optimizing the light-emission efficiency and the power consumption of the LEDs.
Furthermore, if LED blocks having different impedances are connected in parallel relative to the supply voltage, there arises a need to regulate the current using a current regulating unit because the LEDs contained in each group must be driven at constant current, and hence the problem that power loss occurs.
Patent document 1: Japanese Unexamined Patent Publication No. 2009-283775 (FIG. 1)